The invention relates to a method of operating a motor vehicle having at least one drive unit which is suitable for providing a drive torque.
When a load shedding is prompted by the driver of a motor vehicle by releasing the accelerator pedal and is implemented by a spontaneous reduction of the drive torque, this causes a rapid relaxation of the elastic drive train. As a result of the vehicle inertia, a tensioning of the drive train in the opposite direction occurs; i.e. the vehicle drives the engine. By means of this rapid reduction of the drive torque and the bump stop change in the drive train, a pronounced negative engine rotational speed gradient will briefly occur without immediately causing a perceptible vehicle deceleration.
If the vehicle is equipped with a turbocharger, this pronounced negative engine rotational speed, however, has the effect that the air flow delivered by the turbocharger, which, because of inertias of the turbocharger and of the air flow even in this phase does not yet drop significantly, can no longer be absorbed by the internal-combustion engine because of the abruptly reduced rotational speed. This results in excessive pressure conditions or pressure pulsations in the air flow guided between the exhaust gas turbocharger and the internal-combustion engine. This has the effect that the already compressed air flows back in a pulsed manner through the exhaust gas turbocharger in the annular gap between the compressor wheel and the compressor housing toward the intake side. This effect is called exhaust gas turbocharger pumping or surging.
The serious negative effects of exhaust gas turbocharger pumping are manifested mainly by an intense noise development and a massive stressing of the components in the intake air guide.
Since, in the case of modern vehicles, the course of the engine torque during a load shedding (thus, a transition from a driven operation to a coasting operation) can be decisively influenced by a so-called torque guidance former and/or a filtering of the accelerator pedal movement such that a delayed or softer drive torque reduction or drive torque buildup occurs, as a result of a corresponding design or adaptation of the guidance former (forming of the torque jump defined by the accelerator pedal, for example, by a PT1 or PDT1 filter and/or a ramp) and/or by an adaptation of the filtering of the accelerator pedal, the above-mentioned problem can be minimized or even prevented. However, by way of the softer drive torque reduction or drop, a slower vehicle deceleration will also occur. Particularly in the case of sport vehicles, it has to be ensured, however, that, when the accelerator is released, no so-called “pushing-along” of the vehicle will occur, but rather a spontaneous response in the form of a clearly perceptible vehicle deceleration takes place.
Methods of operating a motor vehicle, whereby, for the triggering of a torque increase or torque reduction, the drive torque is influenced by a guidance former, are known, for example, from German Patent Documents DE 10 2005 060 858 A1 and DE 10 2008 054 451 A1.
It is an object of the invention to provide a method of operating a motor vehicle which, in the event of load shedding substantially prevents a so-called exhaust gas turbocharger pumping on the one hand but, at the same time, also ensures a spontaneous response expected by the driver with respect to the vehicle deceleration.
This and other objects are achieved by a method of operating a motor vehicle having at least one drive unit which is suitable for providing a drive torque. A drive torque prompted by the driver by way of the accelerator pedal, in the case of a rapid change of the accelerator pedal position, is filtered by way of a guidance former and/or an accelerator pedal filter (or its rate of change is limited), so that a slower drive torque reduction or drive torque buildup will occur. In the case of a prompted reduction of the drive torque, simultaneously with the slowed-down drive torque reduction, a corresponding triggering of a vehicle brake system is carried out for achieving a specified vehicle deceleration.
The starting point of the invention, in principle, is a known method of operating a motor vehicle having at least one drive unit, wherein a drive torque prompted by the driver by way of the accelerator pedal, particularly in the case of a rapid change, is filtered by way of a guidance former and/or an accelerator pedal filter, or its rate of change is limited, so that a softer or slower drive torque reduction or drive torque buildup will occur. This means that the implementation of the prompted drive torque change is influenced by a guidance former and/or accelerator pedal filter such that, particularly in the case of a prompted drive torque reduction, the drive torque is reshaped in the direction of a softer torque drop. Ideally, an influencing of the drive torque gradient takes place to such an extent that no air flows back in the turbocharger, and therefore the above-described so-called exhaust gas turbocharger pumping is substantially prevented.
However, in order to, at the same time, prevent a pushing-along of the drive unit caused by the slower drive torque reduction, and to be able to ensure a corresponding fast response in the vehicle deceleration, the invention provides that, in the case of prompted reduction of the drive torque, simultaneously with the delayed drive torque reduction, for achieving a defined vehicle deceleration, a corresponding triggering of a vehicle brake system takes place, particularly by an active braking intervention. Via this method, the drive train is therefore only minimally (or not at all) relieved in this critical phase during the load shedding, and the negative engine rotational speed gradient is considerably reduced by the softer torque guidance forming and the corresponding connected lack of a drive train load relief.
Since exhaust gas turbocharger pumping occurs only at very fast drive torque reductions, the vehicle brake system will therefore advantageously be triggered only when the gradient of the drive torque reduction is greater than a specified torque gradient threshold or only during a transition from a drive operation to a coasting operation of the vehicle. Otherwise, the required vehicle deceleration can take place solely by the correspondingly designed or further developed guidance former.
In order to be able to achieve a suitable vehicle deceleration or a vehicle deceleration to be expected by the driver, for example, the desired deceleration, which in turn can be determined from the present accelerator pedal gradient during the release of the accelerator pedal, can be evaluated as the command variable for the required brake management. That is, the vehicle deceleration or the braking torque to be set for that purpose, in an advantageous embodiment of the invention, is defined as a function of the drive torque reduction prompted by the driver and/or by the gradient of the prompted drive torque reduction. As an alternative or in addition, the current dynamics design of the motor vehicle may also be included in the triggering of the brake system, particularly for the determination of the desired vehicle deceleration or of the braking torque to be set for this purpose. If, with respect to its dynamics, the vehicle, for example, has a very sporty design, the vehicle deceleration during the load shedding desired or expected by the driver would therefore be greater than in the case of a vehicle designed for comfort.
A number of advantages are obtained on the basis of the method according to the invention. Therefore, in addition to the above-mentioned advantage, a significant further advantage of the invention is the fact that no additional hardware components in the vehicle are required for the implementation of the method according to the invention, so that the implementation is relatively cost-effective by use of a correspondingly designed software module in the engine control unit. If the braking system is an electric system, the deceleration intervention can also be implemented very easily and efficiently.
Furthermore, the entire drive train will be protected by the clear reduction of load variation peaks. This also results in an increased degree of freedom in the design of the rigidity of the drive train, for example, in the interest of acoustic optimizations of the humming and vibration behavior of the vehicle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.